In decorative films according to the art, appearance of a film surface is likely to be affected by any irregularity of a substrate. To obtain a smooth surface appearance, a substrate must be finished extremely smoothly and this surface adjustment of the substrate requires an extremely long period of time.
Wall paper having both of a foaming layer and a nonfoaming layer has been known in the past. However, most of such products are not directed to absorb any irregularity of the substrate and moreover, due to their structures, they do not exhibit the functions of absorbing irregularity of the substrate and providing a smooth surface.
Japanese Unexamined Utility Model Publication (KOKAI) No. 58-89331 describes wall paper produced by laminating glass paper or a backing sheet comprising a non-woven fabric, a foamable vinyl chloride resin layer, a surface pattern, and optionally, a transparent vinyl chloride sol layer and a transparent acrylic film, in this order. This wall paper is directed to improve staining resistance, and the reference does not mention the thickness of the foaming layer for absorbing irregularity of the foundation. This wall paper is not believed to play the function of absorbing irregularity of the substrate in view of the materials of the backing sheet.
Japanese Patent Publication (KOKAI) No. 60-23019 describes a method of producing a decorative sheet which comprises coating a paste sol containing a foaming agent to the entire surface, or a part, of a substrate, dissolving the paste sol at a temperature at which the foaming agent is not degraded, printing twice thereon a second layer having a low melt viscosity equal to, or lower than, that of the layer containing the foaming agent, printing a third layer, if necessary, and thereafter raising the temperature to the decomposition temperature of the foaming agent and generating foaming. In this case, foaming is exclusively directed to obtain a frosted surface. Therefore, a foaming ratio is low and the function of absorbing irregularity of the substrate cannot be attained.
Japanese Unexamined Patent Publication (KOKAI) No. 452199 describes a decorative sheet obtained by disposing a decorative layer on the surface of a substrate sheet, laminating a first pressure sensitive adhesive layer, an intermediate layer consisting of a soft material (a foaming material) and a second pressure sensitive adhesive layer on the back of the substrate sheet in this order, and further laminating peel paper on the back. In this sheet, the foaming layer as the intermediate layer absorbs irregularity of the substrate. Accordingly, irregularity of the substrate does not extend to the plastic substrate sheet. However, since two layers of pressure sensitive adhesive layers are provided, the structure is complicated and the problem of adhesion remains between the layers. Peel strength is not sufficient, either, and a complicated production process is necessary because the pressure sensitive adhesive must be coated on both surfaces of the foaming layer.